In order to understand regulation of transcriptional activation and/or silencing of a gene, it is important to identify the role epigenetic mechanisms such as transvection an pairing dependent repression play in gene regulation. Although trans-sensing effects have been well documented in plants, flies, mice and man, the underlying mechanism are largely unknown. What all trans-sensing effects have in common is a way of sensing chromosomal homology that is revealed as a change in gene activity. Drosophila provides a superb animal model for studying the gene network required to take an animal from a single cell to a complex multicellular organism and mutations within the bithorax-complex (BX-C) prove an excellent system to study regulation of homeotic genes. Homeotic genes have also been implicated in humans as the cause of some genetic disorders and malignancies. The goal of this study is to determine how chromosome pairing and pairing dependent repression effects the BX-C genes Ultra-bithorax, abdominal-A and Abdominal-B. The BX-C offers several advantages for this proposal. The genetics have been well established, many viable and lethal mutants are available an the molecular structure, including sequence analysis is well known. The relationship between pairing dependent regulation of gene expression will be evaluated on the molecular level based on the hypothesis that the regulatory regions within the BX-C can be subdivided into: (1) regions that are pairing sensitive to the effects of classical transvection (impaired pairing results in a more severe loss of function phenotype and (2) regions that are subject to pairing dependent repression caused by an inability to form proper silencing complexes (abnormal pairing results in more severe gain of function phenotype). Expression levels of BX-C transcripts and proteins will be evaluated in a quantitative manner in Drosophila embryos, imaginal discs and abdominal histoblast in order to study the effects of transvection on a molecular level. In addition, the effects the Pc gene has on pairing dependent silencing will be evaluated. Proteins and transcript levels of the three BX-C genes will be studied in the absence of the PC protein product in the different BX-C mutants to determine how PC protein levels effect pairing dependent repression. Results obtained in these studies will allow us to further understand the role epigenetic mechanisms play in gene regulation and possibly lead to a further understanding of the underlying mechanisms leading to genetic disorders and malignancies caused by a disruption of homologous pairing in humans.